Active spring rate adjust

ABSTRACT

A method of controlling the spring rate of a spring region of an unassembled load beam for a disk drive head suspension by partially etching the spring region and providing the partially etched spring region with a number of bridges which may then selectively opened to achieve a target spring rate. Adjusting the spring rate before assembly of the load beam enables tighter control of resulting performance characteristics of the head suspension. In one aspect, a data set of spring rate reductions corresponding to bridge sets removed is predetermined and used with the corresponding type of load beam to determine the bridges to remove to approximate the target spring rate.

BACKGROUND OF THE INVENTION

The present invention relates to the field of manufacturing beadsuspensions for magnetic disk drives, more particularly, to adjustingthe spring rate for load beams used in such head suspensions, byinitially providing a partially etched spring region with a plurality ofbridges or frets and subsequently adjusting the number of bridges in thespring region of the load beam to reduce the variability in the springrate of such load beams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified side view of a head suspension in an operatingposition above a magnetic disk.

FIG. 2 is a diagram illustrating the effect a wide tolerance on springrate has on force in a head suspension.

FIG. 3 is a diagram similar to that of FIG. 2, except showing the resultof more closely controlling spring rate in a head suspension.

FIG. 4 is a simplified plan view of a load beam useful in the practiceof the present invention and having 7 frangible bridges in a springregion of the load beam.

FIG. 5 is a simplified side view of the load beam of FIG. 4.

FIG. 6 is a simplified plan view of an alternative embodiment of a loadbeam use fill in the practice of the present invention and having 9frangible bridges.

FIG. 7 is a graph showing spring rate versus remaining materialthickness in an etched portion of a 7 fret load beam.

FIG. 8 is a graph showing percent deviation from a target spring rateversus remaining material thickness in an etched portion of a 7 fretload beam.

FIG. 9 is a graph showing spring rate versus remaining materialthickness in an etched portion of a 9 fret load beam.

FIG. 10 is a graph showing percent deviation from a target spring rateversus remaining material thickness in an etched portion of a 9 fretload beam.

FIG. 11 is a graph showing percent frequency variation of a firsttorsional frequency variation versus remaining material thickness in anetched portion of a 7 fret load beam.

FIG. 12 is a graph showing percent frequency variation of a firsttorsional frequency variation versus remaining material thickness in anetched portion of a 9 fret load beam.

FIG. 13 is a pair of graphs showing normal distribution of a 7 fretversion of the present invention, along with the statistical “captureband” of coverage of this version for a population of load beams towhich the present invention is applicable.

FIG. 14 is a pair of graphs showing normal distribution of a 9 fretversion of the present invention, along with the statistical “captureband” of coverage of this version for a population of load beams towhich the present invention is applicable.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the figures, and most particularly to FIG. 1, a simplifiedview of a head suspension 20 may be seen positioned above a magneticdisk 22. Suspension 20 is preferably positioned at a nominal totaloffset distance 24 from a working surface 26 of disk 22. In a finishedproduct, it is to be understood that there is characteristically a“stack up tolerance” in the assembly of parts including the suspension,with respect to the working surface 26. As the nominal total offsetdistance 24 varies according to process and manufacturing variations,the final gram load for the suspension is affected, because of thechange in amount of deflection of the suspension at its “fly height.”This follows from the spring formula:

F=kx  (1)

where force “F” equals spring rate “k” times displacement “x.” Equation(1) is applicable to the present invention in a rewritten form for headsuspensions as:

ΔGL=ΔSR×ΔD  (2)

where the change in gram load “ΔGL” equals the product of the change inspring rate “ΔSR” times the change in deflection “ΔD” caused by changesin nominal total offset distance variations.

Referring now to FIG. 2, the relationship expressed by Equation (2) maybe observed in terms of normal distributions, where the distribution offorce “F” 30 is the result of the product of the distribution ofdistance “x” 32 multiplied by the distribution of spring rate “k” 34. Asshown, the mid points of each distribution represent nominal or “ideal”values. As may be seen a relatively wide distribution of spring rate 34results in a wide distribution of force 30, for a given distribution ofdistance 32.

Referring now also to FIG. 3, it may be seen that the distribution ofgram load GL 36 may be reduced by reducing the distribution of springrate SR 38 for the same distribution of distance or deflection 32. Againthe “ideal” values are indicated to represent the case with novariation.

Returning to FIG. 2, a numerical example to illustrate the effect of thepresent invention is as follows. If the distribution of distance 32 hasa mean value, μ_(D), equal to 2 mm, with one standard deviation, σ_(D),equalling 0.1 mm, and the spring rate distribution 24 has a mean value,μ_(SR), equal to 18 N/m with one standard deviation of spring rate,σ_(SR), equalling 1.5 N/m, then the mean value μ_(F) of the distributionof the force 30 is:

μ_(F)=μ_(D)×μ_(SR)  (3)

and, for this example, μ_(F)=2 mm×18 N/m=36 mN.

The formula for calculating the standard deviation is:

σ_(F)=μ_(F)×[(σ_(D)/μ_(D))²+(σ_(SR)/μ_(SR))²]^(½)  (4)

where σ_(F) is one standard deviation for the force distribution, σ_(D)is one standard deviation for the distance distribution, and σ_(SR) isone standard deviation for the spring rate distribution. In the example,σ_(F)=0.0036 N×[(0.0001 m/0.002 m)²+(1.5 N/m/18 N/m)²]^(½), resulting inone standard deviation of force equal to 0.00035 N or 0.35 mN.

Referring now also to FIG. 3 and keeping the deflection distribution andmean at the same values as given for the distance in FIG. 2, and keepingthe same mean spring rate, but with one standard deviation for springrate tightened to 0.75 N/m, the mean value of the gram load, μ_(F),remains the same as that for the force at 36 mN, since μ_(D) and μ_(SR)have not changed. However, using the present invention, one standarddeviation, σ_(GR), of the gram load distribution, is improved (ascompared to the standard deviation σ_(F) of the force distribution) asfollows: σ_(GR)=0.0036 N×[(0.0001 m/0.002 m)²+(0.75 N/m/18 N/m)²]^(½),resulting in one standard deviation of gram load equal to 0.00023 N or0.23 mN, an improvement of more than 34% over the result for FIG. 2.

It is believed appropriate to characterize the distributions as having a“normal” or gaussian distribution characteristic since such acharacterization is believed to be conservative in that other forms ofdistribution characteristics are expected to result in a more favorablecomparison of the use of the present invention compared to the prior artlacking use of the present invention.

Referring now most particularly to FIGS. 4, 5 and 6, two alternativeembodiments of load beams having the present invention therein arepresented. FIG. 4 is a top view of a first embodiment of a load beam 40with seven bridges 43 located symmetrically about a longitudinal axis 44of the load beam 40 in a spring region 46. FIG. 6 is a top view of asecond embodiment of the load beam 40 with nine bridges 45. FIG. 5 is aside view applicable to either embodiment. In these embodiments, thebridges are located in the spring region 46 between a mounting region 48and a rigid region 50 of the load beam. The spring region 46 has areduced thickness 52, (relative to the material thickness 54 of the loadbeam adjacent the spring region) preferably formed by partially etchingthe load beam in the spring region, and further has material entirelyremoved between respective adjacent bridges.

In the embodiments corresponding to Tables 1 and 2, the bridge widthsare as follows: bridges 1, 2, and 3 are each 0.1 mm wide, bridges 4, 5,6, and 7 are each 0.2 mm wide (for each of the 7 and 9 bridgeembodiments). For the 9 bridge embodiment shown in FIG. 6, bridges 8 and9 are each 0.22 mm wide. In the two example embodiments described herein(the 7 and 9 bridge load beams), all bridges are 0.78 mm long, equal tothe length of the spring region in the direction of longitudinal axis44. However, it is to be understood that the length of the bridges, aswell as other dimensions of the load beams are not necessarily limitedto those specified herein in order to practice the present invention.The unetched material thickness 54 is 0.1016 mm and the reduced(partially etched) thickness 52 is 0.0275 mm. It is to be understoodthat these dimensions correspond to the 7 and 9 bridge embodimentspresented herein, and that the invention may be practiced with otherdimensions.

The use of symmetrical incrementally sized bridges are preferable tomaximize the adjustment range for a given number of bridges. Actualsizes are dependent on the number of bridges and the degree of precisionavailable in the manufacturing process. It is to be understood thatfewer bridges result in less choices, but greater ease ofmanufacturability. Preferred sizes for the spaces between bridges isdependent upon the tooling used to selectively remove bridges to achieveor approach the spring rate desired.

It is to be understood that the load beam 40 is shown in an early stageof manufacturing in FIGS. 4, 5 and 6, and may undergo substantialadditional processing in the course of manufacturing a head suspensionassembly. The present invention is directed to reducing the variabilityof the spring rate of the spring region 46 to reduce the range ofvariation of parameters (particularly gram load) in the final assemblythat are traceable to variability in spring rate of the spring region ofthe load beam.

Referring now to FIGS. 7 and 8, graphs of spring rate versus remainingmaterial thickness (material left in the spring region 46, after partialetching of the load beam) may be seen. FIG. 7 illustrates theimprovement obtained by the present invention in terms of the reductionin variability of actual spring rate in

Newtons/meter. The remaining material thickness, RMT, corresponding toreduced thickness 52 in FIG. 5, is given in millimeters, with a spreadof ±3σ (sigma, or standard deviation) about a nominal reduced thickness52 of 0.0275 mm. The target spring rate for the 7 fret version is 16.25N/m. As may be seen, the 7 fret version of the load beam may be adjustedby removal of frets symmetrically about the longitudinal axis 44 tomaintain the spring rate between 12 and 20 N/m in comparison to theprior art spring rate variability of between 10 and 24 N/m. Curve 60gives the performance of the prior art, while curve 62 illustrates theoperation of the present invention. FIG. 8 shows the same data exceptwith the spring rate expressed as a percentage deviation from the targetspring rate. Curve 64 gives the performance of the prior art, whilecurve 66 illustrates the operation of the present invention. Here thedeviation may be seen to be reduced by the present invention from arange of 80% (−35% to +45%) for the prior art, to a range of less than50% (−25% to +25%). It maybe further noted that 86.6% of the parts shownin FIGS. 7 and 8 are within ±5% of nominal (assuming a normaldistribution). Table 1 gives the actual numerical data for FIGS. 7 and8.

TABLE 1 “As Etched” “As % over Final Final St Dev Target Etched” ASRFrets Removed ASR S.R. (Sigma) Thickness S.R. Orig S.R. Target 1 2 3 4 56 7 Target ASR +3   0.0315 20.9 30.33 45.60 X X X X X X X 16.5 19.51+2.5 0.03084 20.9 28.66 42.43 X X X X X X X 16.5 18.41 +2   0.03017 20.927.004 38.90 X X X X X X X 16.5 17.32 +1.5 0.02950 20.9 25.41 35.06 X XX X X X X 16.5 16.27 +1   0.02883 20.9 23.857 30.84 X X X X X X 16.516.064 +.5  0.02817 20.9 22.384 26.29 X X X X 16.5 16.54 Nominal 0.027520.9 20.94 21.20 X X X X X 16.5 16.16 −.5  0.02685 20.9 19.59 15.77 X XX 16.5 16.4 −1   0.0262 20.9 18.3 9.84 X X X 16.5 16.521 −1.5 0.0255220.9 16.993 2.90 X 16.5 16.44 −2   0.02483 20.9 15.73 −4.90 16.5 15.73−2.5 0.02417 20.9 14.57 −13.25 16.5 14.57 −3   0.0235 20.9 13.45 −22.6816.5 13.45

Adding an additional pair of frets to adjust spring rate gives theresults indicated in FIGS. 9 and 10, which show the same type of dataillustrated for the 7 fret load beam in FIGS. 7 and 8. In FIGS. 9 and 10the target spring rate is 12.5 N//m. In FIG. 9, a variability in springrate for the reduced thickness 52 (without fret adjustment) of thespring region 46 is between 8 and 18 N/m, or expressed in percentages,between about −35% and +45%, an 80% range. In FIG. 9, curve 68 gives theperformance of the prior art, while curve 70 illustrates the operationof the present invention. In FIG. 10, curve 72 gives the performance ofthe prior art, while curve 74 illustrates the operation of the presentinvention.

The present invention allows a reduction to a range of variability ofspring rate between 11 and 13 N/m, or less than ±5% of the desired ortarget spring rate. It is to be noted that substantially all of theparts are within ±5% of the target rate. Table 2 gives the actual datafor FIGS. 9 and 10.

TABLE 2 “As Etched” “As % over Final Final St Dev Target Etched” ASRFrets Removed ASR ASR (Sigma) Thickness S.R. Orig S.R. Target 1 2 3 4 56 7 8 9 Target Target +3   0.0315 20.4 27.741 55.84 X X X X X X X X X12.5 12.190 +2.5 0.03084 20.4 26.192 53.23 X X X X X X X X 12.5 12.486+2   0.03017 20.4 24.668 50.34 X X X X X X X X 12.5 11.735 +1.5 0.0295020.4 23.194 47.18 X X X X X X X 12.5 11.888 +1   0.02883 20.4 21.78943.73 X X X X X X 12.5 11.951 +.5  0.02817 20.4 20.414 39.99 X X X X X XX 12.5 12.240 Nominal 0.0275 20.4 19.089 35.83 X X X X X X 12.5 12.142−.5  0.02685 20.4 17.851 31.38 X X X X X 12.5 12.004 −1   0.0262 20.416.662 26.48 X X X X X 12.5 12.209 −1.5 0.02552 20.4 15.468 20.80 X X XX 12.5 12.105 −2   0.02483 20.4 14.31 14.40 X X 12.5 12.228 −2.5 0.0241720.4 13.252 7.56 X X 12.5 12.298 −3   0.0235 20.4 12.227 −0.19 12.512.227

Referring now to FIGS. 11 and 12, it may be seen that there is aslightly improved resonance variation using the present invention, withthe most pronounced improvements at the “tails” of the ±3σdistributions. Curves 76 and 78 illustrate the performance of the priorart, while curves 80 and 82 illustrate the frequency effects of thepresent invention. It is to be understood that the present invention maybe used to “tune” the first torsional resonant frequency, if desired,rather than be directed to spring rate variability reduction.

To carry out the practice of the present invention, initially a table ofvalues is created for each type or size of product to be manufactured.Table 3 lists the % reduction in spring rate corresponding to everypractical combination of bridges removed, it being understood that“practical” combinations ordinarily will be limited to laterallysymmetrical bridge removal combinations, i.e., the same size bridge isordinarily removed on both sides and at identical distances from thelongitudinal axis to avoid distortion in the static attitude andimbalance in the lateral proportionality of spring rate in the springregion after removal of bridges. Table 3 is thus a data set of valuesfor a particular type or model of load beam, with individual spring ratereductions associated with respective bridge sets removed. As may beseen, Table 3 includes a predetermined minimum increment of 3.2%selectable by including bridge 1 or not.

TABLE 3 Bridges Removed % Spring Rate Reduction 1 3.2 2 + 3 6.4 1 + 2 +3 9.7 4 + 5 13 1 + 4 + 5 16.2 2 + 3 + 4 + 5 19.5 1 + 2 + 3 + 4 + 5 22.84 + 5 + 6 + 7 26.2 1 + 4 + 5 + 6 + 7 29.5 2 + 3 + 4 + 5 + 6 + 7 32.9 1 +2 + 3 + 4 + 5 + 6 + 7 36.2

To carry out adjustment of a particular load beam product, it ispreferred to actively determine the initial or unadjusted spring rate ofeach individual part. This can be done by measuring the remainingmaterial thickness 52 (for partial etched radius products) andcalculating a spring rate from that information, knowing thecharacteristic material from which the part is made. Alternatively, onecan actively directly measure the initial spring rate for each part.This can be done as a process step after initial etching or later in themanufacturing process, for example, after welding additional componentsto the load beam, to account for spring rate variations introduced bysuch additional manufacturing steps.

Once the “As Etched” (initial) spring rate is determined, a comparisonis made with the desired or target spring rate, and a “% over target”value is calculated as the amount (in percent) that the initial springrate is greater than the target spring rate. The percentage is comparedto the values in Table 3 for that product, and an appropriate set ofbridges is selected to most closely match the desired % spring ratereduction needed to adjust the initial spring rate to equal orapproximate the desired or target spring rate. For example, if theinitial spring rate is 20% over the target spring rate for a productcorresponding to Table 3, either of two sets of bridges may be removed.Removing the set of bridges [2+3+4+5] will reduce the spring rate by19.5%, resulting in a slightly high final spring rate, 0.5% over thetarget spring rate. Additionally removing bridge 1 will reduce thespring rate by 22.8%, resulting in a slightly low final spring rate,2.8% under the target spring rate.

It is to be understood that in the practice of the present invention,each type of load beam product would preferably be characterized by itsown particular Table 3. As is readily apparent, the choice of whichbridge set to remove can be easily automated in the production process.

Referring now back to Table 1, at −0.5σ, in this instance the % over ASRtarget was 15.8%. Referring to Table 3, the closest match is the removalof bridge set [1+4+5]. It may be noted that the only options listed inTable 3 are ones that result in a laterally symmetrical part. While itis possible to provide for additional bridge set removals, such setswould result in a laterally non-symmetric spring operation, generallyresulting in unacceptable suspension attributes. It is to be furthernoted that Table 1 indicates removal of bridge set [2+3+4+5+6+7]resulting in a 32.9% spring rate reduction, while removal of bridge set[1+4+5+6+7] would have resulted in a reduction of 29.5%, obtaining acloser approximation to the target spring rate, but with the consequenceof ending up below the target spring rate with the larger reduction.

It is to be understood that the specific spring rate reductionassociated with each particular bridge set removal is preferablydetermined by actual testing to develop the database of respective datasets such as that in Table 3.

In addition to the manufacturing process development and executiondescribed above, it is also of interest to review the increase in radiusstress resulting from the use of the present invention. Turning now toTable 4, the most extreme condition was investigated for radius stress.Table 4 illustrates a condition for the 9 bridge version with all 9bridges removed, thus providing the greatest stress on the radiusregion. In this testing, it may be seen that there is only a slightincrease in radius stress resulting from the implementation of thepresent invention in controlling spring rate in load beams, incomparison to the stress due to etch variation alone.

TABLE 4 Standard Original Stress Present Stress Deviation ThicknessCondition Increase Invention Increase σ mm Max Stress % Max Stress % for0.1 lift* for 1 lift +3 0.0315 5.23 × 10⁴ 11.127 5.33 × 10⁴ 13.139Nominal 0.0275 4.71 × 10⁴  0.000 4.71 × 10⁴  0.000 *Note: The “0.1 lift”is a 0.1 mm back bend at the load point.

Now referring to FIG. 13, it may be seen that using the 7 fret partiallyetched spring region will enable precise adjustment of the spring ratewithin the range of ±1.5σ indicated by the device 80 in the gaussiandistribution 82, “capturing” 86.6% of the population of load beamsindicated by capture band 84, while the remaining population fallswithin the “tails” 86, 88 outside the capture band 84. Referring nowmost particularly to FIG. 14, the 9 fret version will capture over 99%of the product population within a ±3σ range indicated by device 90 indistribution 92. This, in effect, “compresses” the distribution into amuch “tighter” capture band 94 or package, with only very littleremaining in tails 96, 98. Stated another way, it has been observed thatthe 9 fret version the total population is within ±5% of the targetspring rate, or a spring rate variation of 1σ=0.195.

This invention is not to be taken as limited to all of the detailsthereof as modifications and variations thereof may be made withoutdeparting from the spirit or scope of the invention. For example, andnot by way of limitation, it is to be understood to be within the scopeof the present invention to vary the dimensions of the materialthickness and the reduced thickness area of the load beam, and further,either independently, or in combination therewith, to vary the number,size and spacing of the bridges in the spring region of the load beamwithout departing from the practice of the present invention. As afurther example, still not by way of limitation, it is within the scopeof the present invention to store actual values rather than percentagevalues in the respective data sets according to Table 3.

What is claimed is:
 1. A method for controlling the variation in springrate of an unassembled load beam of the type having a longitudinal axiscomprising the steps of: a. partially etching a load beam to provide areduced thickness area in a spring region of the load beam; b. providinga plurality of bridges in the reduced thickness area of the springregion of the load beam; c. determining an initial spring rate of theload beam based on the reduced thickness and bridges in the springregion of the load beam; and d. removing at least one bridge to reducethe spring rate to a desired target spring rate such that the resultingspring rate of the load beam more closely approaches or equals thedesired target spring rate.
 2. The method of claim 1 wherein the step ofpartially etching the load beam further comprises providing an “asetched” original spring rate greater than the desired target springrate.
 3. The method of claim 2 wherein the step of partially etching theload beam still further comprises providing an “as etched” originalspring rate greater than the desired target spring rate by an amount atleast as great as the incremental spring rate adjustment available byremoving one bridge.
 4. The method of claim 3 wherein the bridgesprovided in step b include a range of sizes of bridges, having a minimumand a maximum incremental spring rate adjustments correspondingrespectively thereto, and the original spring rate is greater than aminimum incremental spring rate adjustment available by removing onebridge.
 5. The method of claim 2 wherein the step of partially etchingthe load beam still further comprises providing an “as etched” originalspring rate greater than the desired target spring rate by an amount atleast as great as an incremental spring rate adjustment available byremoving a pair of bridges.
 6. The method of claim 5 wherein the minimumincremental spring rate adjustment corresponds to removing a pair ofbridges.
 7. A method for predicting a reduction in variation of springrate of an unassembled load beam of the type having a longitudinal axiscomprising the steps of: a. partially etching a load beam to provide areduced thickness area in a spring region of the load beam; b. providinga plurality of bridges in the reduced thickness area of the springregion of the load beam; c. determining an initial spring rate of theload beam based on the reduced thickness and bridges in the springregion of the load beam; and d. determining the number of bridges toopen to adjust the spring rate to approach a desired target spring rate.8. The method of claim 7 wherein step d includes using a table of valuescontaining a plurality of incremental spring rate adjustmentsindividually associated with respective bridges.
 9. The method of claim8 wherein step d further includes selecting at least a pair ofsubstantially identical bridges for opening, with each of the bridges onopposite sides of the longitudinal axis of the load beam.
 10. The methodof claim 7 wherein step d further includes making a determination to notremove bridges when the initial spring rate is less than or equal to thedesired target spring rate.
 11. The method of claim 7 wherein step dincludes selecting a single bridge laterally centered in the load beam.12. The method of claim 7 further comprising an additional step a0before step a comprising: a0. forming a set of data pairs in whichindividual predetermined spring rate reductions are associated withrespective sets of bridges removed from the load beam.
 13. The method ofclaim 12 wherein step d further comprises selecting a particular set ofbridges to remove corresponding to a desired spring rate reduction fromamong the data sets formed in step a0.
 14. The method of claim 12wherein the bridge sets are sized to cover a range of about ± onestandard deviation from a nominal spring rate in a normal distributioncentered on the nominal spring rate.
 15. The method of claim 12 whereinthe bridge sets are sized to cover a range of about ± three standarddeviations from a nominal spring rate in a normal distribution centeredon the nominal spring rate.
 16. The method of claim 12 wherein thebridges are sized to permit a predetermined minimum adjustment incrementin the spring rate adjustments obtainable by selection of one bridgefrom among the sets of bridges.